In order to secure a stable operation of a high-tech electronic equipment and protect human life from an electrical accident and an excess voltage by lightning, electrical surge, static electricity, earth fault or the like, an earthing is necessarily required. Because the conventional earthing is performed by burying an earthing rod under the ground, there are many problems of not only constructing costs, time and area but also environmental pollution (especially, soil pollution).
FIG. 1 is a schematic view showing an earthing device of a prior art. As shown in the drawing, a lightning rod 50 and electric, electronic and communication equipment 31a, 31b and 31c are earthed to the ground through earthing rods 50a and 30a. When the electric, electronic and communication equipment 31a, 31b and 31c are provided in plural, they are connected to an MGB (Main Ground Board) 40, and the MGB 40 is connected to the earthing rod 30a. The electric, electronic and communication equipment 31a, 31b and 31c operate by receiving power branching from a panel board 20. The panel board 20 receives power through a transformer 10. The transformer 10 is also earthed to the ground through an earthing rod 10a. Instead of the earthing rod 10a, a bare copper wire (mesh grounding) 32a or a copper earth plate 33a may be used.
However, since the earthing of the prior art is performed by boring or digging in the ground and burying an earth electrode, such as the earthing rod, the bare copper wire or the copper earth plate, under the ground, it requires much construction costs and time and needs a large area for burying the earth electrode. Especially, it is very difficult and takes much cost to bury the earth electrode under the ground and draw out an earth wire in a downtown region due to lots of skyscrapers and limitations of construction area.
Because a soil resistivity is different in each case, the number of the earth electrodes for acquiring a target earth resistance value is varied according to circumstances. The soil resistivity is a very important element in earthing. Generally, the soil resistivity ranges from hundreds of ohm-meter (Ωm) to thousands of ohm-meter (Ωm), and averages between 300 to 1,000 Ωm. Because of such a high soil resistivity, fault current by lightning, electrical surge, noise, static electricity or earth fault is not discharged promptly to the ground, which occasionally causes electrical accidents.
Recently, since a variety of high-tech equipment capable of operating with a low voltage has been increasingly used, a small earth resistance value is required. Therefore, still more earth electrodes should be buried under the ground, which causes problems of larger area for burying the earth electrodes and more construction costs and time.